This invention relates to a method of logging a subsurface formation penetrated by a well bore to determine oil saturation.
The methods of producing oil from underground formations are usually separated into three categories, termed primary, secondary and tertiary recovery methods. In primary production, oil is moved from the formation to a bore hole, and sometimes to the earth's surface, merely by formation fluid pressure. In secondary production, a sweep fluid, such as water or gas, is typically injected in one well to physically push oil through the rock to a producing well. Tertiary recovery typically involves the use of a fluid miscible with both water and oil which is used in front of injected water to remove oil which cannot be removed from the formation by simple water floods. The secondary and tertiary recovery methods are progressively more expensive than primary recovery since they both involve the application of external force and in addition, tertiary recovery requires the injection of expensive chemical compounds into the formation. Before beginning a tertiary recovery project, it is essential to know how much oil was left in the formation after a secondary recovery project was completed.
With the development of pulsed neutron capture logging devices, several basic methods for determining residual oil saturations in a formation have been developed. A description of some of these methods may be found in the article entitled, "Methods for Determining Residual Oil with Pulsed Neutron Capture Logs" by J. E. Richardson, et al, published in the Journal of Petroleum Technology in May, 1973 at pages 593-606. Numerous patents have also been issued for such processes such as U.S. Pat. Nos. 3,748,474; 3,757,575; 3,783,683; and 3,825,752. One of the procedures as outlined in the above referenced Journal of Petroleum Technology article involves taking two pulsed neutron capture logs of the formation when the water salinity has been changed between the logs. This may involve simply running a log with natural formation water present and again after injecting fresh water. But, as is pointed out in above referenced U.S. Pat. No. 3,783,683, where the formation has been previously flooded with water having a different salinity from the natural formation water, mixed salinity water may occur at the point of testing and the first pulsed neutron log should be preceded by an injection of salt water having known salinity. If the first logging step is preceded by salt water injection, this injection will remove any mobile oil so that only residual oil will be present. If the first logging step is not preceded by the salt water injection then the water in the formation when it is logged may have a varying or mixed salinity and the log readings will have little meaning. Also, if the initial salt water injection is not used, the later injection of fresh water will displace mobile oil and the presumption that only the water salinity changes between the two logs will not be valid. Thus it can be seen that this method involving water having two salinities is useful only for measuring the saturation of residual oil; that is, oil in the formation which is not moved by flowing water.
As illustrated in the above referenced U.S. Pat. No. 3,748,474, the two-step logging procedure may be extended to a third log run after the formation has been injected with an alcohol or other sweep fluid which removes all the oil from the formation. This final logging step provides a direct indication of formation porosity which can replace information normally obtained from cores taken in drilling the well or from other logs. This third logging step does not, however, aid in the determination of mobile oil saturation in the formation.
Since in actual practice a secondary recovery waterflood does not remove all mobile oil from the formation, a measurement of only residual oil in the formation does not indicate the true production potential of a tertiary recovery project. In many cases it may occur that the residual oil alone is insufficient to make a tertiary recovery project profitable but when the mobile oil saturation is considered, the project would be profitable. Therefore, to properly evaluate a water-flooded formation for potential tertiary recovery, it is essential that both mobile and residual oil saturations be measured.
Accordingly, an object of the present invention is to provide a bore hole logging procedure for accurately determining both mobile and residual oil saturations in an underground formation.
Another object of the invention is to provide a bore hole logging procedure for measuring reservoir oil saturation in the presence of formation waters having mixed salinities.
A bore hole logging procedure according to the present invention comprises the steps of (1) drilling a test bore hole through the formation using an oil base drilling mud, (2) producing through the test bore hole to remove oil filtrate, (3) running a short range electrical resistivity log and a pulsed neutron capture log, (4) injecting high salinity water into the formation, (5) running a short range electrical resistivity log and a pulsed neutron capture log, (6) injecting fresh water into the formation, and (7) running a pulsed neutron capture log.